Mobile telephones are portable electronic devices used for radio telecommunication over a cellular network of base transceiver stations. Mobile telephones allow users to make and receive telephone calls to and from the public telephone network which includes other mobile devices as well as fixed-line phones. Mobile phones connect to the public telephone network by accessing a cellular network owned by a mobile network operator. Cellular networks enable mobile telephone calls to proceed seamlessly even when the user is moving around wide geographic areas via handoff by base transceiver stations. Mobile telephones also support many additional services and accessories such as short message service (SMS), or text, messaging, electronic mail, Internet access, gaming, camera, and global positioning systems (GPS). Components commonly found on all mobile telephones comprise a rechargeable battery providing the power source and an input mechanism and display to allow the user to interact with the telephone. The most common input mechanism is a keypad, but touch screens and voice activation functionality are found in higher-end mobile phones. Many mobile phones also comprise a subscriber identity module (SIM) on a removable card.